In recent years, sulfur dioxide has been removed from flue gases by various processes. One such process is the so-called Double Alkali process which essentially involves two steps. Initially, the sulfur dioxide in a flue gas is reacted with an alkaline solution of sodium, such as sodium sulfite or sodium hydroxide, in a wet scrubber to produce a sodium bisulfite solution. The sodium bisulfite solution is then reacted with a lime slurry to regenerate the alkaline solution of sodium to produce a calcium sulfite precipitate which is filtered from the alkaline sodium solution and discarded.
As used herein, the term alkaline solution of sodium refers to a solution comprising sodium ions, sulfite ions and hydroxyl radicals which, at a pH of 7-8, may be regarded as a sodium sulfite solution and, at a pH of 10-11, may be regarded as a sodium hydroxide solution.
Heretofore, the regeneration of the alkaline sodium solution has been effected in a cascade of well-stirred, jacketed, batch-mix reactors. The lime slurry and the sodium bisulfite are admitted through separate ports into each reactor and are normally retained in the reactor for relatively-long residence periods, on the order of thirty minutes or so, before being flowed to the next reactor in the cascade. Upon completion of the reaction, the resulting solution is filtered to remove the calcium sulfite which is discarded, and the sodium solution is recycled to the scrubber.
One of the primary disadvantages of the existing regeneration process resides in the large capital costs required to install the cascade of back-mix reactors. For instance, the cost of a typical batch-mix system is about 5% of the cost of the entire scrubbing system. In a power plant, a typical total cost for a sulfur dioxide scrubbing system is about $50,000,000. Hence, the batch-mix system represents a total cost of about $2,500,000.
Another disadvantage resides in reactor operating costs. Customarily, the batch-mix reactors are maintained at a working temperature of about 100.degree. F. to promote the reaction. Thus, depending upon ambient temperatures, a substantial amount of energy is required simply to maintain the batch-mix reactors at operating temperature.